Add comments to clarify interface function caching.
Add comments to clarify interface function caching.

This commit attempts to clarify which functions in the Db interface may be
returning cached data that the InvalidateCache, InvalidateBlockCache, and
InvalidateTxCache functions are used to clear.

davecgh/btcd,toddfries/btcd,Cleawing/btcd,btcsuite/btcd,1d4Nf6/btcd,jcmincke/dcrd,jrick/btcd,ppcsuite/ppcd,jrick/btcd,ppcsuite/ppcd,jongillham/btcd,dan-da/btcd,cjepson/btcd,DanielKrawisz/btcd,Stash-Crypto/btcd,ltcsuite/ltcd,adiabat/btcd,1d4Nf6/btcd,ppcsuite/ppcd-old,jcmincke/dcrd,danoctavian/btcd,toddfries/btcd,stevenroose/btcd,soapboxsys/ombfullnode,DanielKrawisz/btcd,decred/dcrd,dajohi/btcd,Dirbaio/btcd,jcmincke/dcrd,jongillham/btcd,qchain/btcd,jimmysong/btcd,jimmysong/btcd,tb00/btcd,runeaune/btcd,stevenroose/btcd,dan-da/btcd,mathiasrw/btcd,pkothbauer/btcd,ltcsuite/ltcd,sysbot/btcd,Dirbaio/btcd,pkothbauer/btcd,hectorj/btcd,btcsuitereleases/btcd,soapboxsys/ombfullnode,qchain/btcd,Stash-Crypto/btcd,Roasbeef/btcd,danoctavian/btcd,btcsuite/btcd,aakselrod/btcd,Dirbaio/btcd,runeaune/btcd,Cleawing/btcd,davecgh/btcd,jrick/dcrd,BitGo/prova,dajohi/btcd,glv2/ppcd,ppcsuite/ppcd-old,cqswxj2013/btcd,decred/dcrd,Roasbeef/btcd,CryptocurrencyCabal/htcd,CryptocurrencyCabal/htcd,BitGo/prova,cjepson/btcd,FactomProject/btcd,cqswxj2013/btcd,hectorj/btcd,FactomProject/btcd,adiabat/btcd,glv2/ppcd,mathiasrw/btcd,wallclockbuilder/btcd,dajohi/btcd,jrick/btcd,aakselrod/btcd,jrick/dcrd,sysbot/btcd,wallclockbuilder/btcd,btcsuitereleases/btcd,tb00/btcd